I've changed the theme and colors for this example so that the text will stand out.

UPDATE: Regarding the comment, here's one option, though you'll need to play around with the vertical spacing. We first construct exp, a list of plotmath expressions. Then, in annotate, we need y to be a vector of values equal in length to the number of elements in exp. parse=TRUE tells ggplot to treat the elements of exp as plotmath expressions and parse them: